Aluminum brazing is accomplished by heating with a torch or other localized heat source such, by salt dipping, or in a furnace. Furnace brazing can be performed in air using active salts such as zinc chloride, however preferred furnace brazing processes use protective atmospheres in combination with either fluxless braze promoters or non-corrosive fluxes. Various methods of brazing aluminum are known in the prior art. In the context of heat exchanger assemblies, which are characterized by thin aluminum components, brazing has heretofore commonly been effected in the prior art by furnace brazing, most commonly, by controlled atmosphere brazing (CAB) flux and vacuum brazing (VB). Sometimes furnace brazing is used to assemble one set of components then additional components are brazed afterwards using a second brazing operation that may use a localized heating method to avoid damage to the first brazed assembly. To facilitate brazing aluminum, filler metals are commercially available as (1) preforms of wire or shim stock, (2) a paste of flux and filler metal powder, or (3) a clad layer on brazing sheet composite.
In vacuum brazing, the parts to be brazed are provided with sufficient quantities of magnesium, normally present in the filler metal or in the aluminum or aluminum alloy components, such that, when brought to temperature in a brazing furnace under sufficient vacuum conditions, the magnesium becomes sufficiently volatile to disrupt the oxide layer present and permit the underlying aluminum alloy filler metal to flow together. While this technique provides for good brazing, it is essentially a discontinuous process, resultant from the need to apply a vacuum, and thus, is relatively expensive. It is also difficult to control, as it is very sensitive to oxidizing conditions in the furnace atmosphere, and demands that onerous standards of material cleanliness be maintained. Further, the evaporation of the magnesium leads to condensation in the brazing furnace, which requires frequent removal, thereby further adding to costs.
In controlled atmosphere brazing, the ability to braze does not result from mechanical disruption of the oxide but rather, from chemical modification of the oxide by a fluoride salt flux, typically potassium fluoraluminate, which is applied to the parts. As the name suggests, CAB brazing does not require that a vacuum be drawn, such that the process may readily be carried out on a continuous basis, most typically using an inert gas furnace. While this provides for some reduction in cost, this cost saving is partially offset by the necessity for integration of fluxing systems, many of which will suffer from variable flux loading. Moreover, after the flux has been applied, the flux can be susceptible to flaking, such that contamination of the article of manufacture can occur. The flux can also be difficult to apply, especially on internal joints and can cause problems in terms of furnace corrosion and cleanliness in the finished product. More importantly however, it has been found that the flux can lose activity when exposed to magnesium. Thus, this process is not suitable for brazing magnesium-enriched aluminum alloys. As magnesium is a commonly used alloying element in aluminum to improve, inter alia, strength, this reduces the attractiveness of CAB brazing.
Applications for brazing aluminum are not limited to heat exchangers, however heat exchangers require relatively complex assemblies of stacked plates or tubular members that require reliable, low cost joining of multiple joints. Some heat exchangers, for example oil coolers and air conditioning evaporators, require extensive internal joints that must be brazed, in concert with internal passageways that do not provide a source for particulate flux residues in the functional lubrication or refrigerant system. Recently, stacked assemblies of brazed metal plates are being considered as possible methods of assembly of fuel cell engines. Because of their structural similarity to plate-type heat exchangers, heat exchanger brazing technology is of significant interest. The joining of fuel cell plates requires reliable laminar type bonds (extended lap joints). However, fuel cell plates tend to be thin and have intricately formed, narrow fuel field channels which are easily clogged by flux or by excess filler metal flow. In addition, fuel cell systems can be particularly sensitive to ionic species contamination. Using prior art CAB processes, it has been difficult to satisfactorily braze fuel cell plates without internal flux contamination, and therefore CAB is unattractive, and the cost of vacuum brazing is prohibitive. As a consequence, fluxless brazing methods are of increased recent interest, for both heat exchanger and fuel cell engine applications.
An alternative method of brazing aluminum is described in U.S. Pat. No. 3,482,305. In this method, a braze-promoting metal of cobalt, iron, or, more preferably, nickel, is coated on a part to be brazed, in a manner more fully described in U.S. Pat. No. 4,028,200. If properly applied, the nickel reacts exothermically with the underlying aluminum-silicon alloy, thereby presumably disrupting the aluminum oxide layer, and permitting the underlying aluminum metal to flow together and join. Vacuum conditions are not required, such that this method overcomes the limitations of VB. Further, as this method does not require a CAB-type fluoride flux, it is suitable for utilization with magnesium-enriched aluminum alloys, such as are beneficially utilized in heat exchanger construction, and thus, overcomes the drawbacks of CAB. As additional benefits, this process has utility in association with a wide variety of aluminum alloys. However, the bath described in U.S. Pat. No. 4,028,200 provides for relatively slow plating; and has a relatively limited useful life, thereby resulting in significant cost.
Other mechanisms are known in the plating industry as being capable of providing a deposit of nickel upon aluminum. One very popular electroplating bath is the Watts bath, which is known to have some utility in plating decorative nickel on aluminum substrates, provided a surface pretreatment is first carried out. Preferably, a zincate layer is first applied, followed by a thin copper plate (eg. Rochelle-type copper cyanide strike solution) or a thin nickel plate (eg. Neutral nickel strike, nickel glycolate strike), followed by the Watts bath. However, these preplate steps add cost, and in the case of copper, have deleterious environmental aspects, resultant from the use of cyanide. Copper has a further disadvantage in that it can negatively affect the corrosion resistance of aluminum products. Although it is possible to plate nickel directly on the zincate layer, the Watts bath is difficult to control in these circumstances, such that satisfactory adhesion or coverage of nickel is not always obtained. Further, addition of lead to the Watts bath reduces its plating rate, yet further limiting the attractiveness of the Watts bath, given the known benefits associated with the inclusion of lead in the nickel deposit.